1. Field of the Invention
This invention is generally related to processes for the preparation of polymide oligomers and thermosetting polyimides and more particularly to a process for preparing alcohol soluble polyimide precursors which can be imidized in situ and cured through acetylene end groups.
2. Prior Art
Acetylene-end-capped polyimides have been shown to be useful in the formation of structural composites and materials requiring high thermal resistance and good physical properties. Laminates, composites, adhesives and lubricants have been prepared from acetylene end-capped polyimides which exhibit exceptional physical strength, heat resistance and other properties which find many aerospace and commercial applications.
The closest prior art to this invention is found in U.S. Pat. Nos. 3,879,349, 3,928,450 and 3,845,018 issued to the Applicant herein and his co-inventors. This art describes the reaction of diamines, such as 1,3-bis(3-aminophenoxy)benzene with dianhydrides such as benzophenonetetracarboxylic dianhydride to provide anhydride-terminated intermediates which subsequently can be reacted with acetylene-substituted amines to yield acetylene-substituted amide acids (also known as amic acids) or imides or mixtures thereof. The extent of imidization is determined by the amount of heating since the amic acids convert to imides above 50.degree. C. Higher temperatures increase the rate of conversion and thus higher temperatures are usually used. In this art highly polar solvents such as dimethylformamide or N-methylpyrrolidone are generally required to keep the products in solution while the imidization is proceeding. Solution imidization is employed because of its propensity for yielding a more homogeneous reaction.
The necessity for using high-boiling solvents in the prior art processes led to obvious disadvantages such as difficulties in removing the solvent from the final product and the necessity of utilizing high molding pressures and post curing temperatures. The resulting products were therefore expensive because of increased labor and material costs.